How does Allah forgive all people except polytheists, even though the Messenger says: "All of my Ummah are safe, except for those who openly commit sins"?

Allah Almighty clarifies that He does not forgive shirk (associating partners with Him), but He forgives anything less than that to whomever He wills. Thus, the monotheistic believer who dies adhering to faith in Allah and His Messenger is subject to Allah's will regarding forgiveness or punishment, and Allah is not unjust in either case. As for entering Paradise, it is by Allah's mercy and grace, for deeds are means to attain His mercy. The Hadith, "All of my Ummah will be forgiven except those who commit sins openly," indicates that the one who conceals his sin can hope for mercy, while the one who commits sins openly is feared to face punishment and lack of forgiveness; because he belittles Allah's right and forfeits the divine concealment upon himself. Therefore, a servant's open sin is a cause for his destruction and Allah's non-forgiveness of him, whereas a believer's concealment of his sin is a cause for forgiveness, and both are by Allah's will.